1.) Field of the Invention:
The present invention relates to a tension reducer, and particularly to a tension reducer suitable for use in a webbing retractor.
2.) Description of the Related Art:
Webbing retractors are known where a webbing fastened to an occupant is wound up on a webbing take-up spindle by an urging force of a flat spiral spring. Such retractors are provided with tension reducers of a type wherein a tensile force exerted on the webbing is reduced by decreasing the urging force of the flat spiral spring in a state where the webbing is fastened to the occupant in such a manner as to avoid the feeling of applied pressure to whoever wears the webbing.
This type of tension reducer (webbing retractors) may be divided into two types: one having a strong resilient characteristic and the other having a weak resilient characteristic (large and small), depending on the above-described flat spiral springs. One of the two types reduces the tensile force exerted on the webbing by making the urging force exerted on the take-up spindle in a state in which the webbing is fastened to the occupant inactive. This type of tension reducer will now be described in detail.
One end of a weak flat spiral spring is connected to the take-up spindle, while the other end thereof is connected to one end of a strong flat spiral spring via a gear wheel with ratchet teeth formed around the outer periphery thereof. The other end of the strong flat spiral spring is fastened to a case (cover). Namely, the take-up spindle, the weak flat spiral spring, the gear wheel and the strong flat spiral spring are connected in series to one another. The take-up spindle is urged toward a webbing winding direction by these flat spiral springs. In addition, a pawl lever is provided in the vicinity of the gear wheel and can be brought into engagement with the ratchet teeth of the gear wheel to block its rotation. The operation of the pawl lever is interlocked with the activation of a solenoid or the opening/closing of vehicle doors.
After unfastening the webbing from around the occupant, the pawl lever is operated by the above-described operating mechanism so that the rotation of the gear wheel is blocked. As a consequence, the urging force produced by the strong flat spiral spring in the webbing winding direction is not exerted on the take-up spindle, and the webbing is subjected to a state in which the tensile force is applied thereto only by the weak flat spiral spring. Thus, the occupant feels unrestricted by the pressure of the webbing. On the other hand, when the occupant releases the webbing fastened thereto, the pawl lever is operated so that the blocking of the gear wheel is released so as to rotate. Therefore, the urging force produced by the strong flat spiral spring in the webbing winding direction is exerted on the take-up spindle through the gear wheel and hence the webbing is immediately wound up thereon.
In a webbing retractor equipped with such a tension reducer, when the tension reducer is released, i.e., when the pawl lever is operated so that the gear wheel is released from the state of its rotation being blocked, the strong flat spiral spring is abruptly rendered tense so that an unpleasant, dull thud sound is produced.
In order to reduce such a sound developed when a tension reducer is deactivated, a tension reducer (and a webbing retractor) provided with a substantially C-shaped friction spring and an inertial mass body disposed inside the friction spring and moved by centrifugal force has been known.
In this type of tension reducer, a ring-shaped fixed wall (which is fixed securely to a frame supporting the take-up spindle) is formed in association with the outer peripheral wall of the friction spring, and the friction spring having a diameter enlarged by the movement of the inertial mass body by centrifugal force is to be brought into contact with the fixed wall. More specifically, when the tension reducer is deactivated to place the pawl lever at intervals from the gear wheel and the gear wheel starts to rotate by the urging force of the strong flat spiral spring, the inertial mass body is moved by the centrifugal force as the gear wheel is rotated, so as to press the friction spring outwardly toward the radial direction (toward the direction of the diameter to be enlarged). As a consequence, the friction spring is forcibly brought into contact with the fixed wall so that frictional force is produced. This frictional force acts as a braking force relative to the rotation of the gear wheel, that is, the urging force of the strong flat spiral spring, so that the abrupt tension of the strong flat spiral spring is reduced, thus reducing the unpleasant sound.
However, the tension reducer having an inertial mass body and a friction spring and capable of reducing the improper sound developed when released is accompanied by the drawback in that when the urging force of the friction spring is set so as to be small as in a case where the braking force is produced by a slight rotation (low centrifugal force) of the gear wheel, the inertial mass body is moved as the gear wheel is rotated in accordance with the rotation of the take-up spindle, so that the unnecessary braking force is produced, thus deteriorating the feeling of pulling out the webbing and its controllability, even at a time other than that of the deactivation of the tension reducer, i.e., at the time of the normal pulling out of the webbing or at the time of winding up of the webbing. In other words, in order to obtain the feeling of the pulling out of the webbing and its controllability as satisfactory, the braking force for reducing the unpleasant sound cannot be increased and the degree of freedom for setting in the braking force is also reduced.